The TeX compiler then loads a format like plain TeX (which the above commenter uses), LaTeX, or ConTeXt. The format defines what macros are available. LaTeX adds a package system, as does ConTeXt (modules) so you can import even more macros on-demand. These TeX formats differ in scope and thus speed, LaTeX tends to be a bit heavier but what really weighs it down are the myriad of packages it is usually used with.

Many TeX distributions will define aliases like pdflatex in your path such that you can preload pdfTeX with the LaTeX format, but they are not really separate compilers.